1. Field of the Invention
This invention relates to coating materials for semiconductor devices. More particularly, this invention relates to a class of peroxide-curable organosiloxane elastomer compositions useful as junction coatings for silicon diodes. These cured compositions inhibit leakage current under conditions of high inverse voltage and elevated temperature.
2. Background Information
Silicon diodes are used in a variety of electric and electronic devices as rectifiers and for other applications. One type of diode is formed by bonding together substantially equidimensional wafers of N and P types of silicon to form a PN junction. Electrical conductors are then bonded to the silicon wafers. The wafers are subsequently coated with materials, referred to in the art as junction coatings, to protect the sensitive PN junction area against contact with water and other impurities that can substantially alter the electrical properties of the diode. If the diode is exposed to substantial variations in temperature, the junction coating should be sufficiently elastomeric to expand and contract along with the diode without cracking or spalling.
Moisture curable and peroxide curable organosiloxane elastomer compositions are disclosed as junction coatings for semiconductor devices. The polymerizable materials in these coating compositions are polydimethylsiloxanes containing either terminal hydroxyl groups in moisture curable compositions or triorganosiloxy terminal units in peroxide curable compositions. The shortcoming of these compositions is their inabililty to prevent leakage currents at inverse voltages in excess of about 600 volts when the diode is exposed to temperatures of 100.degree. C. or higher. It is believed that the major portion of these leakage currents pass through the junction coating rather than between the two silicon wafers.
The use of moisture curable silicone elastomers as coatings to reduce leakage current in silicon diodes and thyristors is disclosed in Japanese Patent Publication (Koki) No. 70,052/80, which was published on May 27, 1980. In accordance with the teaching of this publication, a mixture that includes a hydroxyl endblocked polydiorganosiloxane, an alkoxysilane and a tin-containing catalyst is applied to a semiconductor device and allowed to cure in the presence of atmospheric moisture.
The application to integrated circuit "chips" of moisture curable silicone elastomers dispersed in organic solvents is disclosed in U.S. Pat. No. 4,017,495, which issued to Jaffe et al. on Apr. 12, 1977. Moisture curable silicone compositions often require a catalyst to promote curing. The presence of these catalysts and residual moisture in the final coating can cause excessively large leakage currents to develop in the semiconductor device.
Another conventional type of silicone elastomer composition is heat curable and contains an organic peroxide as the curing agent. These compositions can be either a liquid or a semisolid "gum" and typically include a polydiorganosiloxane, a filler such as silica and an organic peroxide. The polydiorganosiloxane may contain ethylenically unsaturated hydrocarbon radicals such as vinyl bonded to the silicon atoms of the polymer.
Data in the example that forms part of this specification demonstrate that a commercially available peroxide-curable polydimethylsiloxane elastomer recommended for use as a junction coating composition exhibits a significant leakage current when the device is subjected to an inverse voltage of 1000 volts D.C. at elevated temperatures for an extended time period.
U.S. Pat. No. 3,946,427, which issued to Iwasawa et al. on Mar. 23, 1976, discloses that the molar ratio of methyl to phenyl radicals in a silicone resin used as a junction coating for semiconductor devices should be at least 20:1 to prevent development of leakage currents. Patentees' theory is that the presence of the highly electronegative phenyl radical is responsible for the leakage current observed when a semiconductor device is exposed to temperatures of about 100.degree. C.
Earlier prior art discloses the advisability of including up to 80 percent by volume of phenyl-containing chlorosilanes in organosilane mixtures that are hydrolyzed to yield silicone resins useful as passivation or barrier coatings on semiconductor devices. U.S. Pat. No. 3,684,592, which issued to Chang et al. on Aug. 15, 1972, discloses silicone resin precursors containing from 50 to 100 parts by volume of diphenyldichlorosilane and from 25 to 50 parts by volume of methyltrichlorosilane. The resin is used to form passivation layers on semiconductor devices.
Canadian Pat. No. 835,590, which issued to Lee on Feb. 24, 1970, discloses barrier coatings for semiconductor devices. The coatings contain a methylphenylsiloxane resin represented by the average unit formula R.sub.n SiO.sub.(4-n)/2 where R is methyl or phenyl, n is from 1.1 to 1.75 and the resin contains from 0.5 to 5.0 phenyl radicals per methyl radical.